1. Field of the Invention
The present invention relates to an image coding system, such as a distributed video coding system, having an image encoder and a predictive image decoder.
2. Description of the Related Art
Distributed video coding (DVC) has been attracting considerable attention recently. In a DVC system, Slepian-Wolf techniques are used to decode an encoded image with reference to a predicted image generated by the decoder. In ‘Transform-Domain Wyner-Ziv Codec for Video’, Proc. SPIE Visual Communications and Image Processing, San Jose, Calif., 2004, Aaron et al. describe a DVC system in which the underlying code is a turbo code. Turbo codes are explained by Sklar in Digital Communications: Fundamentals and Applications, 2nd edition, Prentice-Hall, 2001. Low-density parity-check (LDPC) codes can be used in place of turbo codes.
DVC systems in general place a high computational load on the decoder. This is partly because the decoding of turbo codes and LCPC codes is computationally intensive, but another factor is that the generation of predicted images is computationally intensive because of the need to carry out motion compensation on key frames. The key frames are non-predicted frames, inserted periodically in the video sequence, that are encoded and decoded by a conventional coding method such as the method developed by the Joint Photographic Experts Group (JPEG). The decoder predicts other frames from preceding or following key frames.
The computational load could be reduced by uniformly reducing the amount of prediction processing performed, but that would lead to less accurate predicted images. The loss of prediction accuracy would reduce the data compression ratio and further increase the turbo or LDPC decoding load.
There is a need to reduce the amount of processing spent in image prediction while maintaining the accuracy of the predicted images.